Imidazolinone herbicides are used in a wide range of crops where the crop is tolerant of the imidazolinone and weeds in the crop are susceptible. There are also cases where weeds have become resistant to the imidazolinone herbicides. Rice is naturally susceptible to imidazolinone herbicides. However, resistance was developed within a line of rice that led to commercialization of conventional and hybrid rice varieties currently sold as CLEARFIELD (BASF) rice. NEWPATH (imazethapyr) (BASF) can be sprayed on CLEARFIELD (BASF) rice without injury to the plants. This has become a leading method for controlling weedy forms of rice, a weedy form of rice that is not tolerant of imidazolinone herbicides, in a crop of rice that is tolerant to the imidazolinone herbicide. The imidazolinone sprayed on to CLEARFIELD (BASF) rice also controls other weeds, particularly barnyard grass. NEWPATH (imazethapyr) (BASF) is typically applied at the two- to three-leaf and at the 3-4 leaf stage of weedy forms of rice, such as red rice.
A number of problems, however, have developed with the use of the imidazolinones on CLEARFIELD (BASF) rice. They include: some grass weeds have developed resistance to the imidazolinones and are no longer adequately controlled with the use of the imidazolinones; out crossing of the imidazolinone resistant gene into weedy forms of rice is occurring, making the weedy forms of rice less susceptible to the imidazolinone herbicides; and a portion of the CLEARFIELD (BASF) rice F1 crop is dropped into the field at harvest or due to other factors and can become weedy rice forms with partial tolerance to the imidazolinones in future seasons. Additionally, as the herbicides are sprayed via airplanes some areas of the field may receive less than required amounts of the imidazolinone.